In the aircraft industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of the aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, the addition of lithium to aluminum alloys often results in a decrease in ductility and fracture toughness. Where the use is in aircraft parts, it is imperative that the lithium containing alloy have improved ductility, fracture toughness, and strength properties.
With respect to conventional alloys, both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-T7X normally used in aircraft applications. For example, it was found for AA2024 sheet that toughness decreases as strength increases. Also, it was found that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness. Additionally, in more desirable alloys, the combination of strength and toughness would be attainable in an aluminum-lithium alloy having density reductions in the order of 5 to 15%. Such alloys would find widespread use in the aerospace industry where low weight and high strength and toughness translate to high fuel savings. Thus, it will be appreciated that obtaining qualities such as high strength at little or no sacrifice in toughness, or where toughness can be controlled as the strength is increased provides a remarkably unique aluminum lithium alloy product.
It is known that the addition of lithium to aluminum alloys reduces their density and increases their elastic moduli producing significant improvements in specific stiffnesses. Furthermore, the rapid increase in solid solubility of lithium in aluminum over the temperature range of 0.degree. to 500.degree. C. results in an alloy system which is amenable to precipitation hardening to achieve strength levels comparable with some of the existing commercially produced aluminum alloys. However, the demonstratable advantages of lithium containing aluminum alloys have been offset by other disadvantages such as limited fracture toughness and ductility, delamination problems and poor stress corrosion cracking resistance.
Thus, only four lithium containing alloys have achieved usage in the aerospace field. These are two American alloys, AAX2020 and AA2090, a British alloy AA8090 and a Russian alloy AA01420.
An American alloy, AAX2020, having a nominal composition of Al-4.5Cu-1.1Li-0.5Mn-0.2Cd (all figures relating to a composition now and hereinafter in wt. %) was registered in 1957. The reduction in density associated with the 1.1% lithium addition to AAX2020 was 3% and although the alloy developed very high strengths, it also possessed very low levels of fracture toughness, making its efficient use at high stresses inadvisable. Further ductility related problems were also discovered during forming operations. Eventually, this alloy was formally withdrawn.
Another American alloy, AA2090, having a composition of Al-2.4 to 3.0 Cu-1.9 to 2.6 Li - 0.08 to 0.15 Zr, was registered with the Aluminum Association in 1984. Although this alloy developed high strengths, it also possessed poor fracture toughness and poor short transverse ductility associated with delamination problems and has not had wide range commercial implementation. This alloy was designed to replace AA7075-T6 with weight savings and higher modulus. However, commercial implementation has been limited.
A British alloy, AA8090, having a composition of Al-1.0 to 1.6 Cu - 0.6 to 1.3 Mg - 2.2 to 2.7 Li - 0.04 to 0.16 Zr, was registered with the Aluminum Association in 1988. The reduction in density associated with 2.2 to 2.7 wt. Li was significant. However, its limited strength capability with poor fracture toughness and poor stress corrosion cracking resistance prevented AA8090 from becoming a widely accepted alloy for aerospace and aircraft applications.
A Russian alloy, AA01420, containing Al-4 to 7 Mg - 1.5 to 2.6 Li - 0.2 to 1.0 Mn - 0.05 to 0.3 Zr (either or both of Mn and Zr being present), was described in U.K. Pat. No. 1,172,736 by Fridlyander et al. The Russian alloy AA01420 possesses specific moduli better than those of conventional alloys, but its specific strength levels are only comparable with the commonly used 2000 series aluminum alloys so that weight savings can only be achieved in stiffness critical applications.
Alloy AAX2094 and alloy AAX2095 were registered with the Aluminum Association in 1990. Both of these aluminum alloys contain lithium. Alloy AAX2094 is an aluminum alloy containing 4.4-5.2 Cu, 0.01 max Mn, 0.25-0.6 Mg, 0.25 max Zn, 0.04-0.18 Zr, 0.25-0.6 Ag, and 0.8-1.5 Li. This alloy also contains 0.12 max Si, 0.15 max Fe, 0.10 max Ti, and minor amounts of other impurities. Alloy AAX2095 contains 3.9-4.6 Cu, 0.10 max Mn, 0.25-0.6 Mg, 0.25 max Zn, 0.04-0.18 Zr, 0.25-0.6 Ag, and 1.0-1.6 Li. This alloy also contains 0.12 max Si, 0.15 max Fe, 0.10 max Ti, and minor amounts of other impurities.
It is also known from PCT application W089/01531, published Feb. 23, 1989, of Pickens et al., that certain aluminum-copper-lithium-magnesium-silver alloys possess high strength, high ductility, low density, good weldability, and good natural aging response. These alloys are indicated in the broadest disclosure as consisting essentially of 2.0 to 9.8 weight percent of an alloying element which may be copper, magnesium, or mixtures thereof, the magnesium being at least 0.01 weight percent, with about 0.01 to 2.0 weight percent silver, 0.05 to 4.1 weight percent lithium, less than 1.0 weight percent of a grain refining additive which may be zirconium, chromium, manganese, titanium, boron, hafnium, vanadium, titanium diboride, or mixtures thereof. A review of the specific alloys disclosed in this PCT application, however, identifies three alloys, specifically alloy 049, alloy 050, and alloy 051. Alloy 049 is an aluminum alloy containing in weight percent 6.2 Cu, 0.37 Mg, 0.39 Ag, 1.21 Li, and 0.17 Zr. Alloy 050 does not contain any copper; rather alloy 050 contains large amounts of magnesium, in the 5.0 percent range. Alloy 051 contains in weight percent 6.51 copper and very low amounts of magnesium, in the 0.40 range. This application also discloses other alloys identified as alloys 058, 059, 060, 061, 062, 063, 064, 065, 066, and 067. In all of these alloys, the copper content is either very high, i.e., above 5.4, or very low, i.e., less than 0.3. PCT Application No. WO90/02211, published Mar. 8, 1990, discloses similar alloys except that they contain greater than 5% Cu and no Ag.
It is also known that the inclusion of magnesium with lithium in an aluminum alloy may impart high strength and low density to the alloy, but these elements are not of themselves sufficient to produce high strength without other secondary elements. Secondary elements such as copper and zinc provide improved precipitation hardening response; zirconium provides grain size control, and elements such as silicon and transition metal elements provide thermal stability at intermediate temperatures up to 200.degree. C. However, combining these elements in aluminum alloys has been difficult because of the reactive nature in liquid aluminum which encourages the formation of coarse, complex intermetallic phases during conventional casting.
Recent and renewed interest in supersonic transport airplane developmental programs has generated a need for thermally stable, low density, high strength structural aluminum alloys having acceptable levels of fracture toughness. It has been determined that commercially available Al-Cu-Li alloy AA2090 is not suitable for supersonic application. R.J. Bucci et al., in Naval Surface Warfare Center TR 89-106 Report, note that fracture toughness of AA2090 degraded severely after a moderate thermal exposure at 212.degree. F. for about 1,000 hours. In order to achieve the property characteristics suitable for supersonic aircraft structural applications, it is necessary to develop an alloy with good thermal stability at elevated temperatures in the range of 200.degree. F. to 350.degree. F. Moreover, alloys must be developed which also have sufficient physical and mechanical properties for subsonic aircraft structural applications.
In the prior art, Al-Cu based high strength alloys such as AA2219 and AA2519 have been used in elevated temperature aircraft applications. These Al-Cu alloys, however, have only a moderately high strength with a rather high density (0.103 lbs/in.sup.3).
As stated above, the prior art has proposed Al-Cu-Li-Mg-Ag alloy systems for achieving high strength and high stress corrosion cracking resistance among the Al-Li type aluminum-based alloys.
However, the prior art alloy systems discussed above, i.e., Al-Cu based and Al-Cu-Li-Mg-Ag based, exhibit different characteristics in overaging behavior and exposure to elevated temperatures over extended periods of time.
With reference to FIG. 1, differences in age hardening and softening behavior are illustrated between non-lithium containing aluminum-based alloys and lithium containing aluminum-based alloys. The two types of alloys illustrated in FIG. 1 are subjected to increased amounts of thermal exposure, i.e., overaging after artificial aging to peak strengths. During overaging, conventional 7000 series alloys (Al-Zn-Mg-Cu) are represented by the dotted line. These alloys reach peaks strength condition during overaging and, thereafter, additional aging or repeated exposure to elevated temperatures causes these alloys to become softer while at the same time allowing the alloys to recover their fracture toughness. This is indicated by the U-shaped portion of the AA7000 series alloy which curves around and continues upwardly after reaching a given peak strength.
Prior art Al-Li high strength aluminum based alloys are represented in FIG. 1 by the solid line. Once the Al-Li alloy reaches its peak strength by artificial aging, additional exposure to an elevated temperature environment permits the alloy to recover its fracture toughness and ductility only after a severe loss of strength. This is indicated by the broadly shaped curve which, when eventually extending upwardly as the curve for the non-lithium aluminum alloys does, indicates a low strength when fracture toughness recovers.
As such, a need has developed to provide a high strength Al-Li alloy for elevated temperature applications which maintains an acceptable level of fracture toughness throughout thermal exposure to an elevated temperature environment during aircraft or aerospace applications.
Therefore, considerable effort has been directed to producing low density aluminum based alloys capable of being formed into structural components for use in elevated temperature application in the aircraft and aerospace industries. The alloys provided by the present invention are believed to meet this need of the art.
The present invention provides an aluminum lithium alloy with specific characteristics which are improved over prior known alloys. The alloys of this invention, which have the precise amounts of the alloying components described herein, in combination with the atomic ratio of the lithium and copper components and density, provide a select group of alloys which has outstanding and improved characteristics for use in the aircraft and aerospace industry.